1. Field of the Invention
This invention relates to a NOR-type non-volatile semiconductor memory device for writing and erasing data by a tunnel current.
2. Description of the Related Art
A non-volatile semiconductor memory, such as the flash EEPROM, is reaching the limits of a write operation using CHE (Channel Hot Electron) because the power-supply for the memory tends to be of a lower voltage (3.0 or 3.3 volts).
A NOR-type non-volatile memory, which writes and erases data using the FN-tunnel current (Fowler-Nordheim Tunneling Current) with a low voltage power-supply, is preferable to the CHE memory.
The non-volatile memory which writes and erases data using the tunnel current is able to write and erase with a low voltage power-supply or a boosted voltage because little current flows in the memory.
However, the write operation using FN-tunneling has a disadvantage of taking a longer time than the write operation using CHE. For example, while the write operation using CHE takes tens of microseconds, the write operation using the FN-tunneling takes several milliseconds.
In order to solve this problem, for example, it would be possible to write by latching one word line of data from a word line (corresponding to a row line) established in a memory cell, using a single word line as a writing unit, in other words, writing latch data on a page basis. Since the time necessary for writing on a per byte basis could be reduced, the writing time should be comparable to that for the CHE type operation.
However, this leads to a new problem, described below, which occurs in the paginal write operation. After the paginal write operation, a verify operation determines whether or not the data are written in the cell completely and accurately enough to be later correctly read.
Threshold levels for writing for the cells connected to the word line are not at all the same and there is some degree of variation. Therefore, the more often re-writing is undertaken, the wider the variation in threshold levels will become and the more significantly broad the dispersal of such threshold levels for the cells connected to the word line will become.
When the threshold levels for the cells become significantly different, the margins will be insufficient, thereby making the reading itself as well as the control of such reading difficult.